Although they are manufactured in large numbers, each integrated circuit (IC) is unique due to physical randomness, even when the same manufacturing process and the same material are used. The inherent variations can be extracted and used as its unique identification, such as DNA or fingerprints with human beings. Recently, security researchers have proposed different ways to take advantage of physical randomness to build physically unclonable functions (PUFs). One type of PUF is built with an SRAM memory array. A way to build a PUF using SRAM is based on the power-up states of the memory cells in an SRAM.